I generally fit a US 11, UK 10 boot/shoe. I'm in a 27.5 boot. However, length is only one measurements; my boots need to be tiny in the heel and very wide at the ball. Due to getting new boots and then new liners, I had to forego skiing in my work-in-progress boots on my vacation and rent some boots. The rentals were 27.5, not adjusted by punching out the forefoot. My foot is still recovering.
Boots now come in low, medium and large voluem. Get the boots that fit your feet, which may require some work by the fitter. Flex is a matter of personal preference, but generally you should be somewhere between 100 and 150 flex, probably guessing 120 Flex.
Another trick for picking ski length that takes into account all the variables like rocker, flex, purpose, etc. is that any model of ski only comes in particular lengths. If you are in the middle of the ski population with regards to weight and speeds versus other skiers on the hill, pick the length that's in the middle. Faster heavier, go long. Light and slow go short. 2ndary effects for height being far above average bump up a length (longer skis).
It might be better, depending on your budget to get a cheap boot adjusted for your feet to get you through the season, and then buy a better boot next fall, but that's coming from someone for whom rentals just don't work well.